Efforts to improve mineral oil lubricants by the use of oligomeric hydrocarbon fluids have been the subject of research and development for many years and have led to the market introduction of a number of poly alpha olefin (PAO) synthetic lubricants. Significant research on PAOs has been toward developing fluids that exhibit useful viscosities over an extended temperature range while also showing good lubricity, thermal and oxidative stability, and pour point. PAOs generally operate over a wider range of operating conditions than mineral oil lubricants, and may also exhibit lower friction and thus increase the mechanical efficiency of the equipment in which they are used.
PAOs can be produced by reacting an olefin feed in the presence of an acidic alkylation catalyst such as AlCl3, BF3, promoted BF3, a metallocene, or other suitable oligomerization catalyst. PAOs are saturated hydrocarbon compositions and thus generally less polar than mineral oil compositions because the latter are unsaturated and may contain polar moieties. Thus, to improve the solvency and dispersancy of PAO compositions, a polar co-basestock such as an ester or alkylaromatic may be added to the composition. The polar co-basestock, however, could also introduce undesirable side effects. If an ester co-basestock is used, it may result in hydrolytic instability of the composition. If an alkylaromatic co-basestock is used, it may result in poor low temperature or reduced cleanliness properties of the composition.
Alkylaromatic compounds have been known for many years. They possess good thermal and oxidative stabilities, as disclosed in U.S. Pat. Nos. 4,211,665; 4,238,343; 4,604,491; and 4,714,794. These compounds, however, generally have poor rheological properties. Specifically, they have low viscosity indexes (VIs), consistent with their aromatic character. Thus, they are useful as heat transfer and functional fluids due to their good thermal and oxidative stabilities, but are otherwise generally disappointing as lubricants.
U.S. Pat. Nos. 5,254,274 and 5,019,670 disclose methods of improving the thermal and oxidative stabilities of PAOs by alkylating unsaturated oligomers with an aromatic compound. The products have improved stability and solvency due to the aromatics component as well as improved rheological characteristics. U.S. Pat. Nos. 4,737,297; 4,714,794; and 4,665,275 disclose various monoalkylate compounds with good oxidative stability and U.S. Pat. No. 5,342,532 discloses a mono- or dialkylate benzothiophene with good oxidative stability. U.S. Pat. No. 5,177,284 discloses making an alkylated naphthalene fluid with improved thermal and oxidative stability using low alkylation temperatures and low acidity zeolite catalysts. U.S. Pat. No. 5,602,086 discloses blends of alkylaromatics with PAOs to improve oxidation stability, solubility, elastomer compatibility, and hydrolytic stability.
Despite many improvements, current industry trends are demanding even better lubricant performance and in turn adding to the complexity of formulating lubricant compositions. In automotive applications, for example, the trend is toward extending oil drain intervals and improving fuel economy. In industrial applications, the trend is toward increasing oil drain intervals and extending equipment life. New lubricant compositions with improved properties are needed to meet these new performance requirements. Specifically, there is a need for alkylaromatics with an improved balance of thermal and oxidative stabilities, seal compatibility, solvency, and other properties.
Alkylaromatics, and specifically alkylated naphthalenes, are currently marketed in several viscosity grades. Alkylated naphthalenes currently on the market comprise of either: 1) a majority of monoalkylate with some dialkylate or 2) a distribution of mono-, di-, tri-, and higher poly-alkylates. Lower viscosity alkylated naphthalenes, with kinematic viscosities below about 6 cSt at 100° C., generally fall into category 1 and, thus, comprise primarily monoalkylates. Higher viscosity alkylated naphthalenes, with viscosities above about 6 cSt at 100° C., generally fall into category 2 and comprise a distribution of alkylates and a generally lower level of monoalkylates than lower viscosity alkylated naphthalenes.
The different compositions of the lower and higher viscosity alkylated naphthalenes result in a tradeoff of beneficial properties. Whereas lower viscosity grades generally have good oxidation stability, they may be incompatible with some seal materials due to the high level of monoalkylate. This is thought to be due to the porous nature of seal materials, wherein the pores are large enough for lower molecular weight molecules and, thus, lower viscosity grades to penetrate them. This penetration causes softening of the seals and seal swell, both undesirable in lubricant applications. Although higher viscosity grades are less likely to have seal compatibility issues due to their larger size molecules, their oxidative stability and solvency is not as good as lower viscosity grades due to the increased level of alkylation. This increased level of alkylation means the molecules in higher viscosity grades have more positions available for oxidative attack. Furthermore, solvency is generally believed to be a function of aromatic content, with more aromatic content providing improved solvency. Thus, as the level of alkylation increases in higher viscosity grades, the aromatic content decreases and solvency in turn decreases.
A process to selectively synthesize a dialkylate product has been unknown in the art for several reasons. Shape selective catalysts with no acidity on the surface, such as sodium zeolite USY catalysts, have been used to make monoalkylate-rich compounds. Attempting to increase the level of dialkylate by increasing the olefin to aromatic ratio in the feed results in deactivation of the catalyst, presumably due to coking. The effect of this presumed coking may be somewhat overcome by using more catalyst, but very high levels of catalyst are required. Other zeolites with acidity on the surface are somewhat more selective to mono- and dialkylate formation; however, tri- and higher poly-alkylates are still formed and dimerization of the olefin becomes more prevalent.
Alkylaromatic compounds have been produced using Friedel-Crafts alkylation reactions, which involve the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst. The use of non-zeolite catalysts, such as aluminum chloride, clays, triflic acid, or other Brønsted or Lewis acids results in a distribution of mono-, di-, tri-, and higher poly-alkylates. Additionally, dimerization of the olefin can occur with these catalysts as well. Adjusting the stoichiometry in these processes to favor mono- and dialkylates over tri- and higher poly-alkylates results in a large amount of unreacted naphthalene, which is difficult to process. In any process, a distillation step could be applied to separate out mono- or tri- and higher poly-alkylates to yield essentially pure dialkylate, however this would be uneconomical in current processes given the relatively low selective formation of dialkylate in the product stream. Additionally, as molecular weight of the product increases, distillation generally becomes more difficult. Thus, it becomes more difficult to separate the dialkylates from tri- and higher poly-alkylates in the reactor effluent as more of these heavier products is formed.